How do I use an ambient occlusion map in the BGE?
If you don't care much for physical accuracy then you can just multiply the AO map over your albedo texture. Quick and easy.

(I like using nodes but the UI can also be used)



If you do care, though, it gets slightly more complex.

This is the formula we will follow:

Diffuse = (Direct + Indirect) * Color

Direct means material node

Indirect is what is often called Ambient term
It can be approximated as follows:

Indirect = AmbientColor * AO



If you want something a bit better to represent the AmbientColor you could make yourself a pre-convolved environment map (A.K.A. very blurred HDRI)

You might also want to have a UV channel that is used just for AO, separate from the albedo texture. If you do end up using one please, do not use the light-map unwrapping method, it will lower the performance of your game significantly.